Hybrid electric vehicles improve fuel economy by storing energy in a battery during some driving conditions and utilizing that energy to supplement the power of an internal combustion engine in other driving conditions. Furthermore, hybrid electric vehicles can use a smaller internal combustion engine than a comparably sized conventional vehicle because battery power can supplement the power from the internal combustion engine to satisfy short term power requirements such as when accelerating to enter a freeway. Using a smaller engine improves fuel economy because internal combustion engines are typically more efficient when operated at a higher percentage of their maximum power capability.
However, the battery voltage decreases when power is withdrawn. In order to ensure battery with designed operational life, vehicle control strategies typically limit the maximum power withdrawn from the battery to ensure that the battery voltage stays above a predetermined minimum voltage. This maximum battery power limits vehicle performance with a particular engine and limits the opportunity to use a smaller engine.